It was recently discovered that a rare earth-iron-nitrogen based alloy prepared by dissolving an alloy of rare earth metal and iron and nitrogen has excellent magnetic properties. When Sm is used as the rare earth metal, the rare earth-iron-nitrogen based alloy has a uniaxial magnetic anisotropy. With the composition of Sm.sub.2 Fe.sub.17 N.sub.2.1, the rare earth-iron-nitrogen based alloy has high magnetic properties of 470.degree. C. Curie-temperature, 15.4 kG saturation magnetization and more than a 60 kOe anisotropic magnetic field (J. M. D. Coye and H. Sun, J.M.M.M. 87 (1990) L 251.).
The steps mentioned below are followed to manufacture the above-noted rare earth-iron-nitrogen based magnetic material:
(1) preparing a base alloy; PA1 (2) roughly grinding the alloy; PA1 (3) nitriding the alloy; and PA1 (4) pulverizing the alloy (making single-domain particles). PA1 preparing a base alloy; PA1 pulverizing the base alloy; PA1 nitriding the base alloy; and PA1 pulverizing the alloy.
It is known that the rare earth-iron-nitrogen based magnetic material has a strong uniaxial magnetic anisotropy and improved Curie-temperature and saturation magnetization when nitrogen is introduced to the base alloy. In the nitriding treatment step, the rare earth-iron based material powder is treated with heat in nitrogen gas, ammonia gas, hydrogen-nitrogen mixed gas, hydrogen-ammonia mixed gas or the like. In order to keep the coercivity of the rare earth-iron-nitrogen based alloy at a high level, the alloy has to be pulverized to a critical particle dimension of less than 3 .mu.m for single-domain behavior. The alloy is pulverized by a ball mill such as a rotating ball mill, a vibrating ball mill and an attritor or a jet mill (for example, Published Unexamined (Kokai) Japanese Patent Application No. Hei 5-175022, No. Hei 5-304008 and No. Hei 6-45121).
It is important to minimize defects such as processing strains which are formed during the pulverizing process in order to prepare magnetic powder having high magnetic properties. When many defects are formed during the pulverizing process, saturation magnetization is reduced to a great extent, thus decreasing the rectangularity in the hysteresis loop. Rectangularity in the hysteresis loop is Hk divided by iHc (coercivity) where Hk is the outside magnetic field strength when the magnetic flux density at the second quadrant of the magnetic hysteresis loop becomes 90% of the residual magnetic flux density. The rectangularity in the hysteresis loop is preferably close to 1.0. The rectangularity in hysteresis loop has a close correlation to the maximum energy product (BH) max. When the rectangularity in the hysteresis loop is low, no high maximum energy product (BH) max is obtained. Thus, it is necessary to reduce defects that can be formed during the pulverizing process in order to prepare a magnetic powder having high magnetic properties.
However, much stress is added to the crystal structure of the alloy by the above-mentioned ball mill, thus increasing defects and lowering saturation magnetization. A jet mill, though, can pulverize the alloy with little stress to the crystal structure. The pulverizing mechanism of the jet mill is divided into two types, a collision-type and a gas current type. In both types, powder is moved at high speed by pulverizing gas at high pressure. In the collision-type, the powder collides with a target, thus pulverizing the alloy. The powder particles mutually collide with each other and are pulverized in the gas current type. In pulverizing properties, the collision-type is superior to the gas current type. However, in controlling stress added to the crystal structure, the gas current type jet mill is better than the the collision-type jet mill. As a method of pulverizing the above-mentioned rare earth-iron-nitrogen based magnetic material, the gas current type jet mill is effective. However, when pulverizing the rare earth-iron-nitrogen based magnetic material by the gas current type jet mill, powder particles coagulate with each other as the particle diameter becomes small. Therefore, the treatment of pulverizing particles to a critical particle dimension for single-domain behavior by the gas current type jet mill requires a long period of time. The stress that powder particles receive during the pulverizing process accumulates and becomes large as the pulverization time increases. As a result, many defects are formed, and magnetization and rectangularity in the hysteresis loop are reduced even though the rate of reduction is smaller in the gas current type jet mill than the rate in a ball mill method.
Therefore, in order to maximize the advantage of the gas current type jet mill pulverizing method of controlling stress on a crystal structure, the pulverizing efficiency should be improved. Published Unexamined (Kokai) Japanese Patent Application No. Hei 5-175022 discloses the method of improving pulverizing efficiency by pulverizing powder particles while the temperature of the particles is kept at around the Curie-temperature (300.degree.-650.degree. C.) with an air current of high temperature, thus preventing the coagulation of powder particles during pulverizing. However, in this method, a device for keeping the temperature of pulverizing gas high is required, thereby increasing manufacturing cost. Published Unexamined (Kokai) Japanese Patent Application No. Hei 5-304008 discloses the method of pulverizing powder particles to an average particle diameter of 1-3 .mu.m using wet pulverizing treatment, after pulverizing the particles to an average particle diameter of 2.5-10 .mu.m by a jet mill. However, the rare earth-iron-nitrogen based magnetic material is likely to be oxidized, thus reducing magnetic properties. In other word, a medium which can maintain a non-oxidizing atmosphere is required, and mainly an organic solvent is used. There are also problems of safety and cost of equipment for protecting the working environment to consider. Moreover, the wet pulverizing treatment becomes a batch processing, so that productivity is poor. Published Unexamined (Kokai) Japanese Patent Application (J.P.A.) No. Hei 6-45121 discloses the method of reducing the coagulation of powder particles by pulverizing the particles at 0.ltoreq.O.sub.2 .ltoreq.5 vol % oxygen density and with a jet mill so as to cover the particle surfaces with an oxide film. However, since the magnetic properties of the rare earth-iron-nitrogen based magnetic material decline due to oxidation as mentioned above, the method disclosed in Kokai J.P.A. No. Hei 6-45121 is not preferable.